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Discussion Starter #1
guys-

so i'm loosing my clearance between the valve stem and the rocker every time i run the bike....fuel spitting out the carb intake and really choppy everywhere. where should i start and what should i look for along the way?

i have tonight and next sunday to work on the bike before a weekend of races.

thanks so much-
tt
 

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new head? why is that happening? did you ever have room for adjustment? if you have done valve work... your valves may have seated and need to have the stems ground down ever so slightly. otherwise... I pretty much check valve adjustments before every race weekend. make sure the marks are pointing in the right direction... for example on the left hand side, the exhaust shouldd be pointed between 9 oclock and 12 oclock and the intake should pointed between 12 oclock and 3 oclock ... does that kinda clear things up

you could always transplant another stock head... we have 2 weeks

matt

latered
 

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Discussion Starter #3
matt-

yeah, i'm guessing it could be a worn valve stem that's getting longer...but it's been happening after short test rides. i suppose the valve could be warped too, or something's wrong with the seat? i have no real clue. oh, and i do check the valves between each race weekend and have never seen any significant changes.

so what i just did was take the clearance out from .002" to .004" since the .002 has disappeared three times now. i ran the thing around for ten or so minutes at high rpm and it ran on both cylinders and was mostly clean the whole time. it didn't feel very crisp though. it's cooling now and i'll check the clearance again in a bit.

thanks though, i'm thinking the transplant thing is going to be the answer.

-tt
 

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bent valve? although the clearances are usually pretty big initially. and that usually hangs them. but you could have floated it and bent it. id pull the valve and look carefully at the seat, and also the area where the valve sits on the seat. your valves can tell you alot. how about a broken valve spring or something like that? the marks matt is talking about is important.

jc

"tex, if your bikes a cheater, its not a very good one"
 

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Discussion Starter #5
joe-

thanks. the mark on the intake rocker adjuster is in the right spot. on the exhaust, it's pointing down to 8o'clock? the rockers are independent though, right? also, what do the marks mean exactly?

wouldn't a broken spring lead to too much clearance between rocker and stem...instead of less and less?

-tt
 

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Clearance closing up is usually caused by only a couple of things...valve seat receeding or the the valve face tuliping/peening over where it meets the seat.
Many many things can cause the clearance to open up, but for the valve clearance to close up...it's limited....your valve is NOT getting longer...so...either the valve is seating deeper in the seat, or the rocker is moving closer to the valve stem. The only thing that can cause the rocker to move closer to the stem is if the cam lobe or rocker pad is so trashed that it's roughed up and and creating more "lift" at the lobe...this would be just prior to self destruction.
A new head will often loose .004-.006" of clearance as the valve finds it's happy spot in the seat....or if the seat was not cut square to the guide properly and the valve is cocked...it will slowly pound down one side of the seat or valve and lose clearance. I've seen NOS heads with valves that were substantially out of square with the seat. Or if the seats were cut too deep and contact with the valve is moved out towards the edge it will tuplip the edge of the valve and you'll lose clearance.

Typically losing a bit of clearance on a new head/valves/seats is not a huge issue unless it adds up to like .006"+ then keep a close eye on it. If the valves are new then the end up the stem could be getting pounded down a thou or so, a couple thou more for the valve to conform to the seat, a couple thou more for the seat to pound down any high spots....any more than that and something worse is happening.

Or like Joe suggests...a slightly bent valve will open up clearances and then slowly pound itself back into shape somewhat, slowly closing up the clerances as it goes.

May sound silly...but close the intake valve and see if you can get your mouth on the port and blow into it....if any appreciable air at all gets passed the valve then it's a bent/trashed valve or seat.

You can also adjust it and run it a few more times and see what happens....if it's the valve or the seat the damage is already done and it's probably not going to hurt it to see if it stabilizes. If the intake is really passing a lot of air or blowing back through the carb badly...forget it, bent valve...bent enough that if you keep running it you could drop the head off the stem and destroy the engine.

One of the best tools I every bought was a Neway valve seat cutter set, cuts the seats perfectly square to the guides, is fairly easy to use and made a noticable difference in leak down compression on my bikes. About $300 will get you the cutters and tools you need and every couple of years you can freshen up the seats and take very little metal off.
JohnnyB
 

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Tim,

those little marks are to make sure the tip of the rocker arm is as close to the center of the valve as possible. Stock engines those marks are supposed to point away from the center, but on most of the hot engines, we spin them the other way around. You can stick a flashlight in the adjuster hole and watch it move front and back as you turn the eccentric. It wouldn't affect (effect?) your clearances, at least not in in the way your are looking at.

My guess is a bent valve. Valves rotate as the engine runs, so you could be adjusting it and its fine, then you run it and it spins to another spot, where you have no clearance.

When you do John's air check (blowing in the hole), make sure the valve is in the closed position. You can also sometimes see it with a maglight stuck down the intake port, and look through he spark plug hole (with your shop lights off) and look for light.

Last way to check is to stick an air nozzle in the spark plug hole and listen for the leak (same as a leak down with ot the guage telling you how much).

Chris
 

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Discussion Starter #8
guys, thanks a lot-

jb, my car mechanic buddy was floating the idea of the "stretching" stem. he said that he's seen it but i don't understand how it could actually happen. this isn't a new head or valves, so the bent valve seems the most logical to me. the head is still all OEM with no replacements and i know the bike has been getting a little doggier since the end of last season. so i'll try to find a head and switch them out.

thanks again-
tt
 

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Dude, don't swap the whole head, just check that valve and see if it's bent....you want to know that for sure anyway....so you don't do it again. I run OEM Honda heads at .004" valve clearance instead of the stock spec .002"...just for a little extra insurance. I run megacycle or high lift cams at .006-.007".

Of course if it's pounded a spot in the seat you'll have to go with another head or cut the seat. I've bent many a valve and often just thrown in a new one...lapped it in a bit and away I go. Depends on how far up it's bent and if it wore the guide too much. Sounds to me like if it is bent you have a small bend or it would be running on one cylinder.

Tell you car mechanic buddy he's insane. Unless you have like 2000 lb springs you aren't going to stretch a valve stem....you might pull the top of the stem off, but you won't stretch the valve.

Go here: http://www.jrbranson.com/HondaRacer/honda/cylinderhead.htm and take a look at how the contact surface of a valve can get pounded down a few thou. Also...note how thick the valve face is...I've seen OEM valves much thinner...and a seat a bit too wide can peen over the thin edge of the valve and allow it to sink farther in the seat. But that's a bit rare and usually only a problem with serious race springs.

Like everyone else said...I'd bet on a slightly bent valve. If you have the head off and have a head gasket a few thou thicker...might want to use it. Lose a tad of compression but gain some insurance in case of valve float. I'm a big fan of tailoring head gasket thickness for squish and piston/valve clearance.

While you have the head off...take a close look at the piston on the intake side for any sign of contact with the valve....if you see a "shadow" of the the valve on the piston..but no obvious contact...you are pushing the limit as far as clearance.

Of course if you are using stock guides and valves and running it hard you could have just hung a valve and had the piston tap it....or you could even have hung the valve and had the exhaust valve nick it.

If you aren't using synthetic oil....then start.
JohnnyB



Edited by - jbranson on May 29 2007 12:27:59 AM
 

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TT
let me know if you need any spare parts
I have extra valves if you want to try replacing it before you do anything w the head

its informative to read all this info you guys are posting

cheers
 

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Discussion Starter #12
thanks-

it's early and i've got to get to work, but JB. i have been running on one cylinder (all through the FCB race even) until i went ahead and took the clearance out to .004" from .002" last night. then it ran on both and wasn't breaking up anywhere, but wasn't very crisp on the throttle either.

i'll start digging in tonight.


-tt
 

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if one of those 8 things doesnt solve your problem, then youre fucked. start with john and chris' advice. then take apart. and there is no way your valve is stretching. unless its made of lead.



jc

"tex, if your bikes a cheater, its not a very good one"
 

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Discussion Starter #14
guys-

thanks. i'll go through all this advise tonight. joe, it's cool, i've abandoned the magic stretch-armstrong stem idea....it wasn't mine and i didn't like it to begin with, but i just don't know much about the valves beyond their function.

off to blow some head-
tt
 

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that would definitely explain why it ran like dogshit on a hot august day.

jc

"tex, if your bikes a cheater, its not a very good one"
 

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Discussion Starter #16
just to clear my car buddy's name. he did mean that the face could be tuliping which makes the whole valve longer....not my bullshit interpretation that the stem was growing. sorry.


-tt
 

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That makes sense....I should have known you were the dummy not him....just kidding, I love ya TT.

It does happen...had it happen badly on Mary's little 70 single. I've seen a large variation in the valve face thickness on OEM valves. Specially intakes that tend to be thinner at the contact area.

One way to explain it, is imagine putting a valve about 1mm or so undersize in a head....instead of the whole sealing surface contacting the seat...only the outer edge does...this slowly peens over the surface. Had the valve come out of Mary's engine so sharp the edge would cut you, and it was starting to fracture where it was peened over. Most likely causes are over ground valves (making them undersize), over ground seats (making them oversize), bad valve to start with, killer springs on OEM valves or wrong valves installed.

When you get it apart...takes some ultra close up shots of the affected parts...I'm curious what the issue was. If you want to check the valve for bend...chuck it up in a lathe...or even a drill and spin it and see if you can spot any wobble in the head. Hold it by the top 1/2" of the stem.
JohnnyB
 

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Tim,

you can check valve to piston clearance using electrical solder. Take a piece around 4" long, bend the first 1" back over, and twist it so it gains some thickness (use your hands only). Stick the wire in the plug hole when the piston is coming up and the intake valves is closing. Hold it there while you turn over the engine and the valve should squish the wire against the piston. Measure with some calipers and you should have at least 40 thou clearance.

This was taught to me by Stan Lippert and I use it for checking clearance after assembly. There are much more accurate ways of doing it if you are trying to mill heads to a specific squish clearance say, but for general assembly purposes, thsi works very well. Joe has seen me burn up lots of solder-espescially if its someone else's engine I am building.

Also, if you revved your stock engine repeatedly above 11,000rpm, the cast iron guides have been known to weld themselves to the valve stems as they overheat. This is why we use bronze guides in race motors so we can rev to 12 or 13k if needed. this may have bent your valve.

If I am at Summit Friday night and you need help doing a motor or head or valve swap-no problem.

Chris
 

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same thing happened to me on my old 175 . . . now BFD's. Kept closing on the clearance, I kept opening it up. wound up that the valve seat had worked itself loose, and all hell broke loose. It ain't your valve, it's the seat. Pull the head, put in new seats, or get a new (old) head. Soon.
 

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Usually pretty rare to have a problem with an intake seat. Evil, your bike was running a sloper 175 head....exhaust valve seats will drop out of them if they get too hot...usually because of poor contact with the head pipe or lack of a "finned" clamp of some sort. Very important to keep the exhaust port area as cool as possible as it draws the heat out of the exhaust valve seat.

I've used the solder method a few times. Works decent...remember to use a soft solder though. BFD uses a good method...puts in a weak spring to hold up the valve, puts the piston at TDC and uses a dial indicator to measure clearance to contact with the piston...but of course this can't be done on the fly with a fully assembled engine.

Chris....you really use .040" piston to valve? I use .060" as an absolute minimum on exhaust valves. Actually prefer .060 on intake and around .070" on exhaust....but then my little 175 sees 13,000 sometimes. Although I typically tune the clearance by milling the valve reliefs a bit deeper if necessary.
Go here: http://www.jrbranson.com/HondaRacer/CB175/pistons.htm and you can even seem my little .060" note written next to the piston :)
JohnnyB
 
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